There were two issues here:
* cutsom_shape_transform was not properly tagged as not-owned pointer.
* custom_shape_transform and bbone_start/_end could be set to a pchan from
another data-block (not from UI, but RNA access code itself allowed it).
Those two issues were specificaly breaking complex rigs in static
override case.
EDIT: since yesterday, bbone_start/_end are mere accessors to edit_bone
data in RNA, so most of fix related to those was ditched (they still needed
to be 'deactivated' from RNA diffing with PROPOVERRIDE_NO_COMPARISON).
975af239ed